Circuit arrangement that functions as an interface between a SIM card and GSM modem

ABSTRACT

The present disclosure relates to a circuit arrangement and a method for defining the signal direction in a data line between the interfaces of a SIM card and a GSM modem, without a control signal for the direction of data and without remote access. According to the disclosure, the transmission direction of the data signals is detected and as a result only one direction is permitted for the transmission, whilst detection in the opposite direction is simultaneously blocked.

FIELD OF TECHNOLOGY

The present disclosure relates to and apparatus and method for signaldirection definition in a data line between these interfaces.

BACKGROUND

SIM cards have become established as a standard for authentication of auser of a GSM mobile radio. In conventional GSM appliances, such as amobile telephone, the card reader for SIM cards is integrated in theappliance. Further, as the functionality of GSM appliances increases,the importance of the SIM card is also growing since it is no longerused solely for checking the authorization to make telephone calls butfor different applications, such as access authorization to secureareas, that is to say in the end as a key substitute, or for paymentfunctions.

During this process, it has been found that, for some applications, itis desirable for the SIM card reader to be physically separated from theGSM appliance. For example, the SIM card may be inserted in anappropriate card reader in a motor vehicle in order in this way toauthenticate a user for a GSM system in the vehicle. Furthermore, bothin the private field and in the commercial field, a user may beauthenticated via a SIM card reader for telephone systems, computers,network parts, automatic payment devices, or other appliances with GSMfunctionality. In this case, for security reasons, the aim is to providea direct link between the card reader and the GSM appliance for thetransmission of the authentication data and, in applications mentioned,this direct link often has to have a length of several meters, owing tothe physical characteristics.

Generally, however, the electrical drivers for the SIM card interface inGSM modems are designed only for distances of less than 50 cm.

Integrated circuits are admittedly available which allow an “extension”to several meters. However, since the data line to the SIM card isdesigned to be bidirectional, these circuits require a control signalfor the signal direction. However, no such signal is externallyavailable on standard GSM modems. Circuits such as these are thereforedesigned only for direct integration in GSM modems and, furthermore,they are costly.

Another possibility is to use GSM modems with a remote access functionfor SIM data (so-called Remote SIM Access—RSA). In this case, the datais read in and out from the SIM card at a remote point, and isinterchanged between the GSM modem and the SIM card by means of ATcommands (Hayes standard command set for modems, of ATtention). The datamay in this case be transmitted by wire or radio (for exampleBluetooth). However, this solution is costly and, furthermore, dependson the use of GSM modems with RSA functionality, which are commerciallyavailable only in small numbers.

SUMMARY

Accordingly, a configuration is disclosed to provide capability for alink from a separate SIM card reader to a standard GSM modem, whichneither provides any external signal for the data signal direction norhas any RSA functionality. At the same time, this is intended to createthe basis for the capability to advantageously extend the connectingpath to several meters.

Under an exemplary embodiment, a circuit arrangement is disclosed forconnection between the interfaces of the SIM card reader and the GSMmodem, which requires no external direction signal for the data line,since it autonomously identifies the signal direction. When it isintended to transmit signals via one of the two interfaces, the dataline is then blocked in the opposite direction. Furthermore, theelements of the circuit are designed such that they can be subdividedinto two groups, depending on whether it is processing signals from theSIM card reader or signals from the GSM modem.

Under the embodiment, the circuit can be distributed between two chipson the basis of these groups, with a line of the desired length beingconnected between the chips.

Data signal conditioning devices are preferably connected in each of theunidirectional data lines. This makes it possible to improve the qualityof data signals which are adversely affected by being passed on via thelong interface.

The first data signal blocking device, the first data signalconditioning device and the first opposite direction blocking element onthe one hand, and the second data signal blocking device, the seconddata signal conditioning device and the second opposite directionblocking element on the other hand, are each preferably in the form of asingle circuit element. This ensures that the circuit has a compactlayout, ensures that the design is simpler and, in consequence, ensuressimple and low-cost production as well as less susceptibility to faults.

Preferably, the first and the second unidirectional data line have arespective first and second line driver which, in particular, is apush-pull line driver with a low impedance, and which couples therespective data signals to the unidirectional data line. Line driverssuch as these improve the quality of the data signal and reduce theprobability of data losses.

In this case, a first and a second resistor are preferably connected inseries with the respective first and second line driver. These resistorsare used for line matching, to prevent or reduce reflection ofelectromagnetic waves.

The first and the second data signal direction identification device,the first and the second data signal blocking device, as well as thefirst and the second direction identification blocking device arepreferably in the form of comparators, in particular with a thresholdvoltage of 1.5 V. This applies for the conventional 3 volt supplyvoltage in the applicable SIM standard, and comparators are very simplemodules, which carry out the relevant tasks.

In a further embodiment, the elements, which in each case occur inpairs, are subdivided into two groups each having one element of eachpair, with one group being arranged physically close to the SIM card andthe other group being arranged physically close to the GSM modem, andconnecting lines between elements from different groups may have lengthsof up to several meters. It is thus possible to connect a SIM card to aGSM modem that is arranged separated from it by a correspondingdistance, without any supporting functionality for such remote accessbeing required of the GSM modem.

A clock line preferably connects a clock output of the GSM modem to aclock input of the SIM card. In this case, a clock signal improvementdevice and a clock line driver, in particular a push-pull line driverwith a low impedance, is interposed on the clock line which couples theclock signal to an extension line with a length of up to several meters.Clock line drivers and the improvement device ensure that quality lossesin the clock signal are compensated for via a clock line with a lengthof several meters, so that the SIM card and GSM modem can be arranged ata corresponding distance from one another.

A production device and a termination device for an inverted signal areadvantageously connected in parallel with the clock line. The clocksignal can thus be transmitted in a balanced form, thus reducing theemitted electromagnetic radiations. A third and a fourth resistor,respectively, are preferably connected in series with the productiondevice and the clock line driver. These resistors are used for linematching.

A reset line advantageously connects a reset output of the GSM modem toa reset input of the SIM card, with which a reset signal improvementdevice is interposed. This makes it possible to compensate for qualitylosses in the reset signal during transmission, particularly if thereset line is relatively long.

BRIEF DESCRIPTION OF THE DRAWINGS

The various objects, advantages and novel features of the presentdisclosure will be more readily apprehended from the following DetailedDescription when read in conjunction with the enclosed drawings, inwhich:

FIG. 1 shows a block diagram of a circuit arrangement according to anembodiment, wherein five individual lines are used to form the interfacebetween a SIM card 100 and a GSM modem 200; a data line 10, a groundline 50, a supply line 60, a clock line 70 and a reset line 80.

FIG. 2 shows a detail of the circuit arrangement shown in FIG. 1, whichrepresents a first embodiment of the data line;

FIG. 3 shows a detail of the circuit arrangement shown in FIG. 1, whichrepresents a second embodiment of the data line;

FIG. 4 shows a detail of the circuit arrangement shown in FIG. 1, whichrepresents a block diagram of the clock line; and

FIG. 5 shows a complete circuit diagram of the circuit arrangementaccording to the invention as shown in FIG. 1.

DETAILED DESCRIPTION

The bidirectional data line 10 branches into a first unidirectional dataline 30 whose signal direction is from a card data input/output 110 ofthe SIM card 100 to a modem data input/output 210 of the GSM modem 200,and a parallel, second unidirectional data line 40 whose signaldirection is from the modem data input/output 210 to the card datainput/output 110. The line direction of all the direction-specific lines30, 40, 70, 80 is represented by arrows in FIG. 1.

A first direction definition device 120 (which is arranged relativelyclose to the SIM card 100) as well as a second direction definitiondevice 220 (which is arranged relatively close to the GSM modem 200) arearranged in series in the first and in the second unidirectional dataline 30, 40.

The first direction definition device 120 is able to identify thetransmission of a data signal at the card data input/output 110, andthen to block the second direction definition device 220 as well as thesecond unidirectional data line 40 in their forward direction. Inconsequence, when a data signal is being transmitted from the SIM card100 to the GSM modem 200, only the first unidirectional data line 30 canpass the data signal, while data signals in the opposite direction arecompletely blocked. In a corresponding manner, the second directiondefinition device 220 is able to identify the transmission of a datasignal at the modem data input/output 210, and then to block the firstdirection definition device 220 as well as the first unidirectional dataline 30 in their forward direction, so that only the secondunidirectional data line 40 from the GSM modem 200 to the SIM card 100can pass data signals, and data signals in the opposite direction arecompletely blocked.

It is also possible to ensure that the SIM card 100 and the GSM modem200 never transmit data signals via the data line 10 at the same timebut that transmission only ever takes place in one of the twodirections, since the opposite direction is blocked during thistransmission.

The ground line 50 connects ground connections of the circuit elementswhich are arranged in the vicinity of the SIM card 100 to groundconnections of the circuit elements which are arranged in the vicinityof the GSM modem 200. A card ground line 151 carries the groundpotential from a card ground pin 150 on the SIM card to the exterior,and a modem ground line 251 carries the ground potential from a modemground pin 250 on the GSM modem 200 to the exterior. Thus, overall, thisensures that the same ground potential is used throughout the entirecircuit arrangement, as well as in the SIM card 100 and in the GSM modem200.

The supply line 60 connects a card supply pin 160 on the SIM card to themodem supply pin 260 on the GSM modem. On the basis of the current SIMstandard, a voltage of 3.0 volts is provided, although the invention isnot restricted to this voltage. It is usual for the GSM modem 200 tosupply a voltage to the SIM card 100.

The unidirectional clock line 70 connects a modem clock output 270 ofthe GSM modem 200 to a card clock input 170 of the SIM card 100. A clocksignal conditioning device 171 is provided in the vicinity of the SIMcard 100, and a clock line driver 272 is arranged in the vicinity of theGSM modem 200, in order to condition the clock symbol (which losesquality during the transmission through the clock line 70) fortransmission in advance and subsequently.

The unidirectional reset line 80 connects a modem reset output 280 ofthe GSM modem 200 to a card reset input 180 of the SIM card 100. A resetsignal conditioning device 181 in the vicinity of the SIM card 100conditions the reset signal after transmission for the SIM card 100.

The two dashed vertical lines in FIG. 1 indicate one possible way toseparate the circuit elements into two groups, with one group beingphysically associated with the SIM card 100, and the other group beingassociated with the GSM modem 200. The connecting lines which arelocated in the area between the two dashed vertical lines in FIG. 1 maythen be lengthened up to several meters in order to arrange the SIM card100 and/or the associated SIM card reader which is not illustrated, andthe GSM modem 200 at a distance of up to several meters apart from oneanother.

FIG. 2 shows an exemplary embodiment of the data line 10 with theassociated circuit elements in the form of a detail from FIG. 1, alsoillustrating details which cannot be seen in FIG. 1. In this case, thefirst and second direction definition devices 120, 220 are each splitinto three sub-elements, specifically a first and a second data signaldirection identification device 121, 221, a respective first and seconddirection identification blocking device 122, 222, as well as arespective first and second data signal blocking device 123, 223. Thefirst and second data signal blocking devices 123, 223, respectively,which can also block the data signal line in the respective forwarddirection, are connected in the respective parallel first and secondunidirectional data lines 30, 40.

The data line 10 additionally branches from the card data input/output110 into a third parallel branch, which is connected to an input of thefirst data signal direction identification device 121. Its first outputis connected to one input of the second direction identificationblocking device 222, and its output is in turn connected to one input ofthe second data signal direction identification device 221. A secondoutput of the first data signal direction identification device 121 isconnected to one input of the first data signal blocking device 123.

In an entirely analogous manner, the data line 10 from the modem datainput/output 210 also in addition branches into a third branch, parallelto the two unidirectional data lines 30, 40, and connected to one inputof the second data signal direction identification device 221. Its firstoutput is connected to one input of the first direction identificationblocking device 122, and its output is in turn connected to one input ofthe first data signal direction identification device 121. A secondoutput of the second data signal direction identification device 221 isconnected to one input of the second data signal blocking device 223.

In a corresponding manner to that shown in FIG. 1, the circuit elementscan be subdivided into two groups, which are respectively arranged closeto the SIM card 100 and close to the GSM modem 200, in which case lineswhich connect elements from different groups may have lengths of up toseveral meters. These lines are arranged in the area between the twovertical dashed lines in FIG. 2.

A description will now be provided of how a transmission data signalwhich is present at the card data input/output 110 is transmitted to themodem card input/output 210, with the line being blocked in the oppositedirection in this case. The converse situation of transmission from theGSM modem 200 to the SIM card 100 then requires no further explanation,due to the symmetrical arrangement.

In a rest state, all the elements in the direction definition devices120, 220 are deactivated. A data signal which is now produced at thecard data input/output 110 is passed via the third parallel branch tothe input of the first data signal direction identification device 121,which identifies a data signal in the direction from the SIM card 100 tothe GSM modem 200. Via its first output, the data signal directionidentification device 121 then activates the second directionidentification blocking device 222. At the same time, via its secondoutput, it activates the first data signal blocking device 123. Via itsoutput, the second direction identification blocking device 221 blocksthe second data signal direction identification device 222. Inparticular, this also prevents the second data signal directionidentification device 222 from being able to block the first data signaldirection identification device 122 or the second data signal blockingdevice 223 if a transmission data signal occurs at the same time, orlater, at the modem data input/output 210 as well.

The transmission data signal at the card data input/output 110 also inparallel reaches the two unidirectional data lines 30, 40. It is appliedto the first unidirectional data line 30 in the forward direction, andthe second data signal blocking device 223 is deactivated. Inconsequence, the data signal can be transmitted without any impedimentto the modem data input/output 210. It is in the reverse direction forthe second unidirectional data line 40, and in consequence cannot bepassed on here.

Thus, because the first data signal direction identification device 121and the first data signal blocking device 123 are activated at thistime, while the second data signal direction identification device 221as well as the second data signal blocking device 223 are deactivated,any transmission data signal which may subsequently be produced at themodem data input/output can neither activate the second data signaldirection identification device 221 nor can it be transmitted by one ofthe two unidirectional data lines 30, 40. The first unidirectional dataline 30 is reached by it in the opposite direction, and the first datasignal blocking device 123 provides blocking in the secondunidirectional data line 40.

Once the transmission of the transmission data signal has beencompleted, wherein there is no longer a transmission data signal at thecard data input/output, all of the activated circuit elements in thedata line 10 are deactivated until a transmission data signal once againoccurs on the SIM card 100 or at the GSM modem 200.

Thus, overall, this makes it possible to ensure that the SIM card 100and the GSM modem 200 never transmit data signals via the data line 10at the same time. If transmission data signals occur virtually at thesame time both at the card data input/output 110 and at the modem datainput/output 210, then the minimal time or line speed differences whichare always present mean that one of the two direction definition devices120, 220 will block the other, so that the data signals can neverthelessstill be transmitted only in one direction until the current datatransmission process has been completed.

FIG. 3 shows a second embodiment of the data line 10 with the associatedcircuit elements as a detail from FIG. 1. Only those elements whichdiffer from those in the first embodiment as illustrated in FIG. 2 willbe described.

A first data line driver 132 is connected in the first unidirectionaldata line 30 in the vicinity of the SIM card 100, and a data signalconditioning device 231 (which is referred to in the following text asthe “second”) is connected in the first unidirectional data line 30 inthe vicinity of the GSM modem 200. Analogously, a second data linedriver 242 is connected in the second unidirectional data line 40 in thevicinity of the GSM modem 200, and a first data signal conditioningdevice 141 is connected in the second unidirectional data line 40 in thevicinity of the SIM card 100. In this case, the first and second datasignal conditioning devices 141, 231 together with the respective secondand first data signal blocking devices are in the form of respectivelyintegral first and second conditioning/blocking elements 130 and 240.

The function of the data line drivers 132, 242 is to convert the datasignal to a signal which is suitable for transmission over a relativelylong data line with a length of up to several meters, while the datasignal improvement devices 141, 231 convert the data signal back again,after this transmission, to a data signal format which can be understoodby the SIM card 100 and by the GSM modem 200. This solves the problemthat data signals can be transmitted over only a short distance of up tohalf a meter in conventional SIM card readers and GSM modems.

FIG. 4 shows a detail of the circuit arrangement shown in FIG. 1,illustrating the block diagram of the clock line additionally with thosedetails which cannot be seen in FIG. 1.

A clock line driver 271 and a balancing device 272 are connected inseries in the clock line 70 in the vicinity of the GSM modem 200, and abalancing cancellation device 172 as well as a clock signal conditioningdevice 171 are connected in series in the clock line 70 in the vicinityof the SIM card 100. The function of the clock line driver 271 and ofthe clock signal cancellation device 171 is the same as that of thecorresponding elements in the data line 10, that is to say thepreparation and conditioning of the clock signal for covering a linelength of several meters.

The balancing device 272 balances the clock signal for transmission viathe clock line, while the balancing cancellation device 172 converts thebalanced clock signal back to a clock signal with a single polarityagain. The balanced transmission has the advantage that it reduces theemitted electromagnetic radiation, since the electromagnetic waves canat least partially cancel one another out outside the clock conductor.

FIG. 5 shows a complete circuit diagram of the circuit arrangementaccording to the invention, from FIG. 1. In this case, and in contrastto FIG. 1, the illustration shows not only functional blocks butconventional individual circuit elements, in order to illustrate oneexemplary embodiment of the invention. Wherever possible, the followingtext uses the same reference symbols as above, when the circuit elementsare analogous to the functional elements which have already beenintroduced above. In this case, completely identical elements will notbe described again.

The general configuration relating to the data line 10, the ground line50, the supply line 60, the clock line 70 and the reset line 80 is alsoshown in FIG. 5. In the area which is bounded by the two dashed verticallines, the lines are in the form of extension lines, and may havelengths of up to several meters.

The card data input/output 110 and the modem data input/output 210 (whenin the function of being an output) are in the form of an open collector(or an open drain), in accordance with the ISO standard A card collectorresistor 111 is thus connected between the supply line 60 and the carddata input/output 110, and a modem collector resistor 211 is connectedbetween the supply line 60 and the modem data input/output 210. The twocollector resistors 111, 211 are of the order of magnitude of 3-20 kΩ,with the modem collector resistor 211 generally already being integratedin the GSM modem.

The data line 10 branches into the parallel first and secondunidirectional data lines 30, 40, and into a third parallel branch. Inthis case, inverter gates are respectively provided as first and seconddata line drivers 132, 242 in the unidirectional data lines 30, 40. Thedata line drivers 132, 242 may be gates based on HCMOS technology (Highperformance Complementary Metal Oxide Semiconductor), but this choicedepends on the line capacity to be driven. A first data driver resistor133 is connected in series with the first data line driver 132, and asecond data driver resistor 243 is connected in series with the seconddata line driver 242. These data driver resistors are used for linematching, that is to say they prevent or reduce the reflection ofelectromagnetic waves in the conductor, and they ensure that thegradient of the signal flanks is limited, thus ensuring reduced emittedradiation, that is to say a better EMC behavior (ElectromagneticCompatibility).

A comparator, which is connected as an inverting threshold voltagecomparator, is in each case provided as the integrated first and secondconditioning/blocking element 130, 240, that is to say it is at the sametime used as the first and the second data signal blocking device 123,223, respectively, and as the first and the second data signalconditioning device 141, 231, respectively. The first or the secondunidirectional data line 30, 40, respectively, is connected via arespective series resistor 134, 244 to the one, inverting input of thesecomparators 130, 240, and the threshold value voltage is applied to thesecond, non-inverting input, via in each case two resistors 124 a, 124 band 224 a, 224 b, which are connected to ground 50 and to the supplyvoltage 60 as a voltage divider. If the resistors 124 a, 124 b and 224a, 224 b have the same magnitude, then they result in the thresholdvalue voltage being set to half the supply voltage, in particular to 1.5volts for a supply voltage of 3.0 volts. If the comparators have opencollector outputs, as, in particular, in the case of LP339 comparators,then their outputs can be connected, as illustrated, directly to thecard or modem data inputs/outputs 110, 210.

The comparators 130, 240 at the same time also define the forwarddirection of the unidirectional data lines 30, 40, that is to say theyare respectively used as the first and second opposite directionblocking elements.

The first and second data signal direction identification devices 121,221 are likewise in the form of comparators. The data line 10 is in eachcase connected to the non-inverting input of the first and second datasignal direction identification devices 121, 221 via a respective firstor second identification device resistor 125, 225. The threshold valuevoltage is applied to the second inverting input via in each case tworesistors 126 a, 126 b, or 226 a, 226 b, which are connected as avoltage divider to ground 50 and to the supply voltage 60. If theseresistors 126 a, 126 b and 226 a, 226 b are also of the same magnitude,then they set the threshold value voltage to half the supply voltage, inparticular to 1.5 volts for a supply voltage of 3.0 volts.

Upstream of the non-inverting input and downstream from the output ofthe first or the second data signal direction identification device 121,221, respectively, the supply line 60 is connected to the third branchof the data line 10 via a respective resistor 127 a, 127 b or 227 a, 227b. The outputs of the first and the second data signal directionidentification devices 121, 221 is connected via first and secondrespective diodes 128, 228 to the inverting input of the respectivefirst and second integrated conditioning/blocking element 130, 240, andvia the extension line to a non-inverting second and first respectivedirection identification blocking devices 222, 122.

In this case, the first and second direction identification blockingdevices 122, 222 are also in the form of comparators, and theirrespective second inverting input in each case receives the thresholdvoltage via the voltage divider 124 a, 124 b or 224 a, 224 b,respectively, while their outputs are connected to the inverting inputof the first and second respective data signal direction identificationdevices.

The operation of the circuit arrangement for the transmission of a datatransmission signal from the modem data output/input 210 to the carddata output/input 110 will be described in the following text. Forsymmetry reasons, no further explanation is required for a datatransmission process in the opposite direction. The procedure when datais transmitted at the same time in both directions has already beendescribed above in conjunction with FIG. 2.

In the rest state, the modem data output/input 210 is at a HIGH levelowing to the pull-up resistor 211 which is integrated in the GSM modem200. A soon as the GSM modem 200 starts to transmit, this is signaled bya LOW level. This leads to a LOW level at the output of the second datasignal direction identification device 221. This LOW signal is producedvia the second diode 228 at the inverting input of the secondimprovement/blocking element 240, and leads to the first unidirectionaldata line 30 being blocked. No data signal can therefore be transmittedfrom the SIM card 100.

The LOW level at the output of the second data signal directionidentification device 221 also acts via the extension line on thenon-inverting input of the first direction identification blockingdevice, and whose output then produces a LOW signal. At the invertinginput of the first data signal direction identification device 121, thisresults in blocking of the non-inverting input; the output of the firstdata signal direction identification device 121 remains at a HIGH level,irrespective of the non-inverting input. Although a data transmissionsignal from the card data input/output 110 is now applied to thenon-inverting input of the first data signal direction identificationdevice 121, it cannot produce a LOW level at the output:

The first data signal direction identification device 121 is blocked.

The HIGH level of the output of the first data signal directionidentification device 121 is passed via the extension line to thenon-inverting input of the second direction identification blockingdevice 222, and its output remains open (open collector). The seconddirection identification blocking device 222 can therefore not block thesecond data signal direction identification device 221 which hasinitiated the direction definition process.

The data line 10 can thus pass data signals only in the direction fromthe GSM modem 200 to the SIM card 100, and the circuit elements on thedata line 10 have the same functionality for direction definition andfor carrying the data in a better manner, which has been described abovein conjunction with FIGS. 1 to 3.

The ground line 50 has the same configuration as that described inconjunction with the block diagrams relating to FIG. 1.

The only additional item in the supply line 60 is the capacitor 61,which is connected to ground and is used for decoupling the operatingvoltage from the line conditions. In accordance with the applicable SIMstandard, the supply voltage is 3.0 volts, although the invention is notrestricted to this. The resistance of the supply line 60 should be aslow as possible.

The clock line driver 271 for the clock line 70 is in the form of asimple gate. The clock signal conditioning device 171 is likewise in theform of a gate, although this gate may also be omitted if the powercapacities are relatively low, for example in the case of a relativelyshort extension line.

The balancing device 272 is in the form of a branch in parallel with theclock line, with the clock signal being inverted by a gate 273 in onebranch. On the opposite side, in the branch with the inverted clocksignal, a resistor 173 a and a capacitor 173 b are used as atermination, in fact with the two parallel branches being joinedtogether again. This forms the balancing cancellation device 172 on theopposite side. Since the clock signal frequency is relatively high(approximately 1-4 MHz), the reduced emitted electromagnetic radiationresulting from this balanced signal transmission is desirable.

A first and a second clock line resistor 276, 277 are respectivelyconnected in series with the clock line driver 271 and with theinverting gate 273 in the balancing device 272. These resistors are usedfor line matching, in an entirely analogous manner to the first andsecond data driver resistors 133, 243.

The signal can be transmitted without a separate driver on the resetline 80. The reset signal improvement device 181 comprises a comparator182, which receives its threshold value voltage from the voltage divider124 a, 124 b, and which is connected at the output via a reset resistor183 to the supply line. In the following text, a gate 184 is alsoconnected in series with the output of the comparator 182. The resetsignal improvement device 181 may also be omitted if the powercapacities are relatively low, for example in the case of a relativelyshort extension line.

The circuit arrangement according to the invention thus conditions SIMsignals at the GSM modem, and converts them back, at the SIM cardholder, to signals which are compatible with the SN card, and viceversa. The signal conditioning process takes account, in particular, ofthe EMC requirements for emitted radiation characteristics.

At the same time, when a data signal is being transmitted, the circuitarrangement defines the transmission direction so that it is notpossible to transmit in both directions at the same time: The end whichis active first of all locks itself and renders the opposite endinoperative.

The circuit does not take any account of protective measures againstelectrostatic discharges (ESD). However, those skilled in the art willbe familiar with how elements could be added for such measures.

While the invention has been described with reference to one or moreexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiments disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. An interface apparatus between a SIM card and a GSM modem,comprising: a bidirectional data line connecting a card datainput/output of the SIM card to a modem data input/output of the GSMmodem; a branching area within said bidirectional line, wherein saidbranching area comprises a parallel first and second unidirectional datalines, each respectively comprising a first and a second oppositedirection blocking element, such that the first unidirectional data linecan pass data signals only in the direction from the SIM card to the GSMmodem and the second unidirectional data line can pass data signals onlyin the opposite direction; a respective first and second data signalblocking device operatively coupled to the first and the secondunidirectional data line, wherein the first and second data signalblocking device can in each case block data signals in the forwarddirection; a first and a second data signal direction identificationdevice operatively coupled to the card and modem data input/output;wherein the respective first or second data signal directionidentification device is operatively coupled to the respective second orfirst data signal direction identification device via a respectivesecond or first direction identification blocking device; and whereindata signals are transmitted in only one direction so that thetransmission of data signals from the SIM card is identified by thefirst data signal direction identification device, which then uses thesecond direction identification blocking device to block the second datasignal direction identification device in the direction from the GSMmodem to the SIM card, and the first data signal blocking device blocksthe data line in the direction from the GSM modem to the SIM card; orwherein data signals transmitted from the GSM modem is identified by thesecond data signal direction identification device, which then uses thefirst direction identification blocking device to block the first datasignal direction identification device in the direction from the SIMcard to the GSM modem, and uses the second data signal blocking deviceto block the data line in the direction from the SIM card to the GSMmodem.
 2. The interface apparatus as claimed in claim 1, wherein a firstdata signal conditioning device is connected in the secondunidirectional data line, and a second data signal conditioning deviceis connected in the first unidirectional data line.
 3. The interfaceapparatus as claimed in claim 2, wherein the first data signal blockingdevice, the first data signal conditioning device and the first oppositedirection blocking element on the one hand, and the second data signalblocking device, the second data signal conditioning device and thesecond opposite direction blocking element on the other hand, are eachin the form of a single circuit element.
 4. The interface apparatus asclaimed in claim 1, wherein the first and the second unidirectional dataline have a respective first and second push-pull data line driver,which couples the respective data signals to the unidirectional dataline.
 5. The interface apparatus as claimed in claim 4, wherein a firstand a second data driver resistor are connected in series with therespective first or second data line driver.
 6. The interface apparatusas claimed in claim 3, wherein the first and the second data signaldirection identification device, the first and the second data signalblocking device, and the first and the second direction identificationblocking device are comparators.
 7. The interface apparatus as claimedin claim 6, wherein the comparators have a threshold voltage of 1.5volts.
 8. The interface apparatus as claimed in claim 1, wherein, of theelements which occur in pairs, one is in each case arranged physicallyclose to the SIM card and the other is arranged physically close to theGSM modem, and connecting lines are provided between them.
 9. Theinterface apparatus as claimed in claim 8, wherein the connecting lineshave a length between 20 cm and several meters.
 10. The interfaceapparatus as claimed in claim 1, wherein a clock line is connected to amodem clock output of the GSM modem to a card clock input of the SIMcard, wherein a clock signal improvement device and a clock line driverare arranged on the clock line, and wherein the clock line drivercouples the clock signal to an extension line.
 11. The interfaceapparatus as claimed in claim 10, wherein a production device and atermination device for an inverted signal are connected in parallel withthe clock line, in order to allow a balanced clock signal transmission.12. The interface apparatus as claimed in claim 11, wherein a first anda second clock line resistor, are respectively connected in series withthe production device and the clock line driver.
 13. The interfaceapparatus as claimed in claim 1, wherein a reset line connects a modemreset output of the GSM modem to a card reset input of the SIM card,with which a reset signal improvement device is interposed.
 14. A methodfor signal direction definition in a data line between the interfaces ofa SIM card and a GSM modem, comprising: transmitting data between theSIM card and the GSM modem via separate transmit and receive lines,wherein the signals on the transmit and receive lines are combined insaid SIM card interface and said GSM modem interface, respectively;identifying transmission data signals on the SIM card and at the GSMmodem; after identification of a transmission data signal on the SIMcard, blocking the transmission of data signals from the GSM modeminterface and furthermore blocking the identification of data signals inthe GSM modem interface; after identification of a transmission datasignal at the GSM modem, blocking the transmission of data signals fromthe SIM card interface and furthermore blocking the identification ofdata signals in the SIM card interface; and connecting a first datasignal conditioning device to the receive line, and a second data signalconditioning device to the transmit line.
 15. The method as claimed inclaim 14, wherein the transmit and receive line each have respectivefirst and second push-pull data line driver, which couples therespective data signals to the unidirectional data line.
 16. The methodas claimed in claim 14 wherein the transmit and receive lines have alength between 20 cm and several meters.
 17. The method as claimed inclaim 14, further comprising the step of connecting a clock line to amodem clock output of the GSM modem and to a card clock input of the SIMcard, and improving a clock signal on said clock line by a clock signalimprovement device and a clock line driver arranged on the clock line,wherein the clock line driver couples the clock signal to an extensionline.
 18. The method as claimed in claim 17, further comprising the stepof connecting a production device and a termination device for aninverted signal in parallel with the clock line, in order to allow abalanced clock signal transmission.